1. Field of the Invention
This invention relates to a light emitting device.
2. Background Art
A white LED with high emission efficiency realizes an illumination device that can replace incandescent lamps and fluorescent lamps and facilitates low power consumption. Furthermore, a blue-violet semiconductor laser device with high emission efficiency facilitates enhancing the performance of DVD (digital versatile disc) and other optical disc drives.
These light emitting devices are made of nitride semiconductors. In the case where the active layer thereof has a multiple quantum well structure, the piezoelectric effect due to lattice mismatch tilts the energy band of the barrier layer and the well layer. In the tilted energy band, electrons are likely to be spatially distant from holes in the well layer.
When carriers are injected into the multiple quantum well, it is likely that there are more electrons than holes in the well layer located at one end of the multiple quantum well, and more holes than electrons in the well layer located at the other end thereof.
Thus, the multiple quantum well made of nitride semiconductors has a problem of decreased emission efficiency due to low carrier recombination probability.
JP-A-2006-013463(Kokai) discloses a technique related to a nitride semiconductor light emitting element which includes a light emitting layer having a multiple quantum well structure with relaxed compressive strain. In this technique, each of the barrier layers, which sandwich the light emitting layer on both sides, contains impurity throughout the layer, and the impurity concentration in the center portion along the thickness direction is higher than in the portion in contact with the well layer. It is stated that this enables high emission intensity.
However, even in this technique, the energy band bending throughout the multiple quantum well layers constituting the active layer is small, and insufficient for transferring carriers so that they are efficiently confined in the multiple quantum well layers. In particular, holes, which have a large mass, are more difficult to migrate than electrons. Hence, it is not easy to enhance emission efficiency.